Something

function ajouter_slashes($texte)
{
      if(get_magic_quotes_gpc() === 1)
      {
              $texte = stripslashes($texte);
      }

      return mysql_real_escape_string($texte); //une connexion à mysql doit exister avant
}


$req='UPDATE user SET ';
$c=0; 

if(isset($_POST['user_nom']))
{
     $user_nom = ajouter_slashes(htmlentities($_POST['user_nom']));
     $c++;
     $req.='user_nom="'.$user_nom.'"';
}

if($c>0)
{ 
    $req.=' WHERE user_pseudo="'.$_SESSION['pseudo'].'"';  
    include 'classe_mysql.php'; 
}